A variety of electronic devices such as portable computers, portable phones, personal digital assistants, and other portable and non-portable electronic devices may utilize a rechargeable battery. In some instances, the rechargeable battery may be deeply discharged. A deeply discharged battery may be defined as a battery having a battery voltage level less than an under voltage threshold level where the value of the under voltage threshold level may vary with battery type and manufacturer. To avoid stressing such a deeply discharged battery at the start of a battery charging process, a relatively small “wake-up” current may be provided to the deeply discharged battery. The wake-up current may be provided to the battery until the battery voltage level reaches the under voltage threshold level. At that time, the rechargeable battery may no longer be considered a deeply discharged battery and a larger charging current may safely be provided to the battery.
A DC to DC converter and its associated controller may regulate the charging conditions for the rechargeable battery. In order to provide a signal representative of the charging current to the controller of the DC to DC converter, a sensor may be coupled to the path providing a charging current to the rechargeable battery. A sense amplifier may also be utilized in conjunction with the sensor to amplify the signal provided by the sensor. However, a conventional sense amplifier utilizes only an n-type input sense amplifier that will not work when the rechargeable battery is deeply discharged and the wake-up charging current is required. This is because the voltage of the rechargeable battery drops to a value low enough to be out of the common mode voltage of the n-type input sense amplifier.
Since the n-input type sense amplifier will not work when the rechargeable battery is deeply discharged, the conventional sense amplifier does not provide any charging current signal representative of the wake-up charging current to the controller of the DC to DC converter. Rather, upon sensing a deeply discharged battery, the controller disregards any signal from the sense amplifier and rather provides a particular control signal over a particular range to control the charging current when the battery is deeply discharged. For example, the controller may provide a pulse width modulated (PWM) signal to the DC to DC controller having a duty cycle between 0 and a certain value. However, such a conventional method results in a wake-up charging current that may have an inaccurate mean value, may have a large peak to peak ripple current, and may suffer from noise.
Accordingly, there is a need for a sense amplifier that may be utilized even when the rechargeable battery is deeply discharged.